An easy test is to open up a terminal window and run the htop program. It will show you if a process is using up all of the CPU. If there is a process using up all of the CPU then we can try to focus in on that. If the CPU is loafing then the problem is probably in the cooling system. Most laptops should be able to cool themselves even when their CPU is maxed out, so regardless of the htop results I suspect a problem in the cooling system. When a laptop or even a desktop starts to overheat when it wasn't overheating before, the first thing I look for is dust clogging up the cooling or a similar problem.